Free cutting alloys



"Patented Jan. 7, 1936 PATENT OFFICE FREE CUTTING ALLOYS Louis W. Kempf and Walter A. Dean, Cleveland, Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application September 18, 1933, Serial No. 689,879

2 Claims. (o1. 75-1 This invention relates to aluminum base alloys and it is especially concerned with improved alloys of this nature containing magnesium and silicon as the principal alloy components.

5 Aluminum-magnesium-silicon alloys of the type described in U. S. Patent 1,472,739 to Jefiries and Archer have been widely used because of their excellent hot and cold working characteristics combined with satisfactory strength and ductility. The alloys referred to contain between about 0.5 and 4 per cent each of magnesium and silicon and are generally heat-treated in the manner described in the aforesaid patent to develop the maximum strength. Alloys containing as little as 0.1 per cent of each of these two elements have also been used successfully for certain purposes, notably, electrical equipment. Wrought or extruded articles made from these alloys often require some machining operation such as boring, drilling, planing, shaping, and the like, to'complete them for their intended use. This type of alloy has a somewhat better machining quality than other aluminum base alloys and for this reason aluminum-magnesium-silicon alloys of the kind described have been often selected where this property is of importance. In spite of this relative superiority, however, these alloys do not have the degree of machinability desired for large scale or automatic machine production. For purposes of this nature the alloy should be free cutting or free machining, that is, the chips should flow freely from the article being cut, they should also be short and breakable to prevent fouling of the cutting tool or operating parts of the machine. A bright, smooth, evenly cut surface-should be left on the machined article. The cutting speed should also be greater than for ordinary aluminum alloys in order to promote more rapid and economical production of machined articles. These ends should be accomplished without undue wear on the cutting tool which would necessitate frequent resharpening or replacement. -Aluminum-magnesium-silicon alloys of the class herein disclosed 45 do not possess a machinability-of this order with a. consequent limitation in the field of their utility.

-It is an object of our invention to improve the machining quality-of the above-mentioned alloys 50 by rendering them free machining. Another object is to accomplish the foregoing purpose without substantially affecting the physical properties of the alloy.

'Our'invention is predicated upon the discovery 55 that the addition of small amounts of lead, biselements may be used in the alloys in the proportion of from about 0.05 to 6 per cent, the

amount to be employed in a particular case being dependent on the properties desired and the severity of the cutting operation. For the purpose of our invention the elements lead, bismuth, and thallium are substantially equivalent and 10 are considered to constitute a class of substances because of their similarity in behavior in the alloys herein disclosed in respect to the machining quality of said alloys. These elements are referred to, therefore, as free machining elements 15 or constituents.

The amount of lead, bismuth, and thallium that may be added to an aluminum-magnesiumsilicon alloy can be varied to meet different conditions as indicated above, the amount of each 20 element that may be employed for this purpose ranging from about 0.05 to 6 per cent. For many applications we prefer to add from about 1 to 4 .per cent of the free machining constituents to the alloy in question. This preferred range for 2 the added elements is particularly effective within the limits of composition usually observed in making the alloys, namely, from about 0.5 to 2 per cent of magnesium and from about 0.75 to 2 per cent of silicon. An alloy within this range 30 which is frequently employed for such varied articles as rolled sheet and forged crank-cases for radial airplane engines of the internal combustion type, is one containing about 0.6 per cent of magnesium and about 1 per cent of silicon,

the balance usually being substantially aluminum. The machinability of this alloy is greatly improved by the addition of about 3 per cent of lead without substantial detriment to the working or physical properties. Small breakable chips 40 are removed from this alloy leaving a smooth pleasing surface. The susceptibility of the alloy to thermal treatment does not appear to suffer from the presence of the free machining elements. Bismuth or thallium in like amount also improve the machining characteristics of the alloy. g I

It has furthermore been found. that lead, bismuth, and thallium may be advantageously combined in aluminum-magnesium-silicon alloys and that a degree of machining quality is frequently obtained which is superior to that of the same alloy but containing an equivalent amount of only one free machining element. For example, an alloy containing about 0.6 per cent of magnesium, 1.0 per cent of silicon, 2 per cent of lead, and 2 per cent of bismuth, the balance being substantially all aluminum, cut more readily and had a better appearing machined surface than an alloy containing the same proportions of magnesium and silicon but with 4 per cent bismuth present. The total amount of added free machining elements should not exceed about 6 per cent and preferably not more than about 4 per cent or less than 1 per cent should be used. The relative proportions of the elements may be varied, but we have found that satisfactory results are obtained by using them in about equal parts. By virtue of this discovery regarding the joint action of two or more of the free machining elements it is possible to employ a smaller amount of added constituents to obtain a desired degree of machinability than if only one element. is used. In some applications it is necessary to limit the amount of added elements in the base alloy, and our discovery makes it possible to meet such conditions in an acceptable manner.

The tensile properties of the alloy are not affected to any marked extent by the addition of up to about 4 per cent of the free machining elements. In larger amount there often is a slight decline in strength but this is compensated for by a somewhat better machinability.

The lead, bismuth and/or thallium may be most convenientLv added to the molten aluminummagnesium-silicon alloy in solid metalic form since the melting points of these elements is considerably below that of the aluminum alloy and the temperatures commonly used in melting the same. If more than about 1.5 per cent of these 5 elements is to be added to the alloy, the temperature should be raised above that usually employed in melting practice, the heavy metals added, and the molten mass stirred vigorously to assure a uniform mixture of the alloy. The method of 1 adding-these melts to aluminum referred to here is more fully described in co-pending application Serial No. 689,885, filed September 18, 1933.

The term "aluminum as used herein and in the appended claims embraces the impurities 15 usually found in aluminum ingot of commercial grade or picked up in the course of the handling operations incident to ordinary melting practice.

We claim:

1. An aluminum base alloy containing from an about 0.5 to 4 per cent of magnesium, from about 0.5 to 4 percent of silicon, and from about 1 to 4 per cent each of lead and bismuth, the balance being aluminum.

2. An aluminum base alloy containing about 0.6 25 per cent of magnesium, about 1 percent of silicon, about 2 per cent each of lead and bismuth, the balance being aluminum.

' LOUIS W. KEMPF.

WALTER A. DEAN. l0 

